Z-index values on nav won't work

I have image div I've placed in my sidebar - on which I added position: absolute and a z-index of 33. This pushes the image over any of the page content and nav bar.
I want my nav items to appear OVER this image, and I thought a simple position: relative, z-index 34 would do it. However this isn't working and my nav remains covered by the image.
Is there some rogue jquery or other code that would prevent me from creating a simple z-index on the nav?

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